Modern electric power generating stations include steam turbine power generating systems comprised of a boiler, steam turbines, and an electric generator. The boiler produces high pressure saturated steam that is usually superheated in tubes fired in the boiler. The steam turbines are connected in series trains with steam flowing from turbine to turbine. The high pressure superheated stream from the boiler is fed into the inlet of the upstream steam turbine. Steam pressure and temperature decrease as the steam moves downstream through the turbine train. The steam turbines are connected to a drive shaft that turns the electric generator producing electric power. The steam boiler is fired with a fossil fuel, e.g. natural gas, coal or lignite, or is heated by a nuclear reactor. Modern fossil fuel fired boilers typically operate at pressures between 1800 and 2400 psig and some operate above the critical pressure of water, which is 3206 psia. Nuclear powered boilers typically operate at much lower pressures, about 600 psig.
Typically, a steam side stream is extracted from one of the intermediate downstream turbines in the turbine train at a pressure and temperature significantly lower than the high pressure steam raised in the boiler. The extracted steam side stream is reheated and then fed back into the steam turbine train at a point downstream of the extraction point.
The exhaust steam from the steam turbines is condensed. The steam condensate is preheated, and then recycled to the steam boiler. Treated boiler feed water is added to the steam condensate to makeup losses. Typically, the boiler feed water stream is preheated with steam extracted from an intermediate point on the steam turbine train.
The capital cost of steam turbine generating systems per KWH generating capacity is high. But they are thermally efficient and have low fuel cost. Accordingly, steam turbine systems are cost effective when operated continuously to provide base load power.
Modern power stations also typically include gas fired turbine units that drive electric generators. Gas turbines cost less than steam turbine units per KWH of power capacity but they are less energy efficient than steam turbine units. Accordingly, gas turbine generators are best suited for intermittent operation to meet peak power duties.
Combined cycle units are an increasingly important component of modern power generating stations. Combined cycles are comprised of a gas turbine-generator unit wherein the hot exhaust gas from the turbine is fed into a boiler to raise steam. The steam powers a condensing steam turbine that drives a power. Alternatively, the steam is used for process heating.
Firing temperatures of gas turbines are being increased as turbine construction materials are improved to withstand higher operating temperatures. Increasing firing temperature increases gas turbine fuel efficiency. Accordingly, combined cycles are now competitive against steam cycles for base load power generation.
Minimizing fuel consumption is a key objective in design and operation of electric power generating stations. Reducing fuel consumption reduces fuel cost and reduces the amount of carbon dioxide and other pollutants dispersed into the atmosphere, or in the case of nuclear reactors, reduces nuclear fuel cost and nuclear wastes to be disposed. Fuel efficiency of a generating system in the power industry is commonly expressed as the heat rate for the system which in English units is defined as the BTU's (British Thermal Units) of heat from combustion of fuel required per KWH (kilowatt hour) of electricity produced. The heat rate can be expressed either at the lower heating value (LHV) which means that water vapor produced by total combustion of the fuel is not condensed or at the higher heating value (HHV) which means that the combustion water is condensed.